A major restriction in the development of a working Rapid Single Flux Quantum (RSFQ) logic circuit with high-Tc superconductors is given by the influence of thermal noise. This gives reason to ask for a general determination of the digital bit error rate. As other approaches, our method of calculating the switching probability is based on the Fokker-Planck equation. In the past few years the bit error rates for a single Josephson junction, SQUIDs and the comparator were calculated by using this theory. We demonstrate numerical solution of the multidimensional Fokker-Planck equation to calculate bit error rates due to thermal noise for a Toggle Flip Flop circuit. In the present work, we combine thermal noise analysis with the effects of process variations in order to derive rules for designing high-Tc RSFQ logic circuits